This application is a continuation of U.S. Ser. No. 15/218,145, filed Jul. 25, 2016, entitled, “STATIC DISCHARGE SYSTEM”, which is a continuation of U.S. patent application Ser. No. 14/921,161, filed Oct. 23, 2015, now U.S. Pat. No. 9,425,195, entitled, “STATIC DISCHARGE SYSTEM”, which is a continuation of U.S. patent application Ser. No. 13/537,443, filed Jun. 29, 2012, now U.S. Pat. No. 9,245,879, entitled, “STATIC DISCHARGE SYSTEM”, incorporated by reference herein in their entirety. This application is related to copending U.S. patent application Ser. No. 13/537,407, filed Jun. 29, 2012 (Attorney Ref. 2024/7) entitled “Switching Device with Charge Distribution Structure”, incorporated by reference herein in its entirety.
This invention is related to semiconductor devices. In particular, the invention is related to heterostructure field effect transistors (HFETs) that operate as switches.
High voltage semiconductor switches are key components in electronic circuits for power conversion. Examples of these applications include power supplies for electronic equipment, drives for electric motors, and inverters for solar cells.
A power switch has an on state that allows the device to conduct current, and an off state that prevents the device from conducting current. When in the on state, a power switch may conduct tens or hundreds of amperes while the voltage across the switch is less than one volt. When in the off state, the power switch typically must withstand hundreds or thousands of volts while conducting substantially zero current. The voltage that the device can withstand in the off state while conducting no more than a given small value of current is sometimes referred to as the breakdown voltage.
High voltage HFETs are fabricated to have, among other properties, a breakdown voltage that is predictable and stable. The fabrication of high voltage HFETs may include special structures of conductive components surrounded by an electrically insulating material. Such insulated conductors are sometimes called floating conductors because their electric potentials do not have a well-controlled relationship to a reference electric potential value under static conditions. The floating conductors may change their potentials in a controlled way under dynamic conditions.
Capacitive coupling between the conductive components within the electrically insulating material may allow the net total electric charges to distribute more or less uniformly within a region of the device in response to rapid changes in voltage at the terminals of the device. The distribution of charge during voltage transitions may prevent high local electric fields that can reduce the breakdown voltage. Use of charge distribution structures may also allow designers of the device to make accurate predictions of breakdown voltage before fabrication.
A perfect insulating material surrounding the conductive components of the charge distribution structure would prevent the conductive components from acquiring a static charge from the environment. Electric currents leaking through an imperfect insulating material can cause charge to accumulate on the conductive components of the charge distribution structure.
Electric fields from accumulated static charge can interfere with the ability of the charge distribution structure to prevent the high dynamic electric fields that reduce the breakdown voltage of the device. Since the accumulated static charge is the result of past electrical stress on the device, the breakdown voltage may depend on the device's history.
A solution is required to prevent the accumulation of static electric charge on floating conductors in semiconductor devices.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
FIG. 1 is an electrical schematic that shows an equivalent circuit of an example static discharge system in accordance with the teaching of the present invention.
FIG. 2 is an electrical schematic that shows an equivalent circuit illustrating the general configuration of an example static discharge system accordance with the teachings of the present invention.
FIG. 3 is a cross section of a semiconductor structure that shows transistors of an example static discharge system in accordance with the teachings of the present invention.
FIG. 4 is a top view of a semiconductor structure that shows transistors of an example static discharge system and a portion of an example high voltage power HFET with an example charge distribution structure.
FIG. 5A is an illustration in a perspective view that shows relative positions of various parts and features of an example semiconductor device that includes a static discharge system and a portion of a power HFET in accordance with the teachings of the present invention.
FIG. 5B is an illustration in a different perspective view of the example semiconductor device in FIG. 5A illustrating the relative positions of other parts and features not visible in the view of FIG. 5A.
FIG. 6A is an illustration in perspective view of the example semiconductor device in FIG. 5A with passivation removed to reveal metal that is not visible in the illustration of FIG. 5A.
FIG. 6B is an illustration in a different perspective view of the example semiconductor device in FIG. 5B illustrating the relative positions of other parts and features not visible in the view of FIG. 5B.
FIG. 7 is an example flow diagram illustrating generally a sample flow of operations in a process to construct the example semiconductor device illustrated in FIG. 5A and FIG. 5B that includes a static discharge system in accordance with the teachings of the present invention.
FIG. 8A is an illustration in a perspective view that shows relative positions of various parts and features of another example semiconductor device that includes a static discharge system and a portion of a power HFET in accordance with the teachings of the present invention.
FIG. 8B is an illustration in a different perspective view of the example semiconductor device in FIG. 8A illustrating the relative positions of other parts and features not visible in the view of FIG. 8A.
FIG. 9A is an illustration in perspective view of the example semiconductor device in FIG. 8A with passivation removed to reveal metal that is not visible in the illustration of FIG. 8A.
FIG. 9B is an illustration in a different perspective view of the example semiconductor device in FIG. 8B illustrating the relative positions of other parts and features not visible in the view of FIG. 8B.
FIG. 10 is an example flow diagram illustrating generally a sample flow of operations in a process to construct the example semiconductor device illustrated in FIG. 8A and FIG. 8B that includes a static discharge system and a portion of a power HFET in accordance with the teachings of the present invention.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.
Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. Particular features, structures or characteristics may be included in an integrated circuit, an electronic circuit, a combinational logic circuit, or other suitable components that provide the described functionality. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.
The present invention includes a network of transistors that is coupled to the conductors in a charge distribution structure of a semiconductor device. The transistors of the network allow electric charge to accumulate on the conductors when the semiconductor device is in an off state, and the transistors automatically reduce the accumulated charge on the conductors when the semiconductor is in an on state.
FIG. 1 is electrical schematic diagram 100 that shows an equivalent circuit of an example static discharge system 102 in accordance with the teaching of the present invention. In the circuit of FIG. 1, transistor QHV 150 is a high voltage HFET that includes components of an internal capacitively-coupled charge distribution structure 174. Transistor QHV 150 is an HFET with a drain terminal 152, a gate terminal 156, and a source terminal 158.
The internal capacitively-coupled charge distribution structure 174 of high voltage HFET QHV 150 is represented by conductive components E1 128, E2 122, and E3 116 that are surrounded by an electrically insulating material 104. An example physical construction of an example high voltage HFET that includes a charge distribution structure is presented later in this disclosure.
FIG. 1 shows capacitors with dashed lines within the electrically insulating material 104 to represent capacitance between the individual components of the charge distribution structure 174 and between those components and other parts of the transistor QHV 150. It is appreciated that capacitance exists between every pair of objects that can possess an electric charge. The representation in FIG. 1 highlights the capacitance that is discharged by the static discharge system 102 according to the teachings of the present invention.
Capacitance CD,3 112 represents the capacitive coupling between the drain terminal 152 of transistor QHV 150 and component E3 116. Capacitance C3,2 118 represents the capacitive coupling between component E3 116 and component E2 122. Capacitance C2,1 124 represents the capacitive coupling between component E2 122 and component E1 128. Capacitance C1,0 represents the capacitive coupling between component E1 and the gate terminal 156 of transistor QHV 150.
FIG. 1 also shows resistors with dashed lines within the insulating material 104 to represent the finite resistance that exists between the individual components of the charge distribution structure 174 and between those components and other parts of the transistor QHV 150. Since electrical insulating material 104 is imperfect, electric charge is able to move within the material in response to electric fields that correspond to difference in voltage. The movement of charge between conductors within the insulating material in response to a difference in voltage between conductors is sometimes referred to as leakage current.
Resistor RD,3 114 represents the resistance between the drain terminal 152 of transistor QHV 150 and component E3 116 of the charge distribution structure 174. Resistor R3,2 120 represents the resistance between component E3 116 and component E2 122 of the charge distribution structure 174. Resistor R2,1 126 represents the resistance between component E2 122 and component E1 128 of the charge distribution structure 174. Resistor R1,0 represents the resistance between component E1 128 and the gate terminal 156 of transistor QHV 150. The value of the resistance between conductive components of the charge distribution structure 174 and the terminals of the transistor QHV 150 is determined by the resistivity of the insulating material 104 and by the physical separation between the conducting components and the terminals.
Transistor QHV 150 in the example of FIG. 1 operates as a switch in a circuit that includes a constant voltage source VB 108 and a resistor R 106. In typical applications, voltage source VB may have a value of several hundred volts. In the example of FIG. 1, voltage source VB 106 has a negative terminal coupled to a common return 110 and a positive terminal coupled to one end of resistor 106. The other end of resistor 106 is coupled to the drain terminal 152 of transistor QHV 150. The source terminal 158 of transistor QHV 150 is coupled to the common return 110. In the example of FIG. 1, a variable voltage source 160 that produces a switching control voltage VGS 162 is coupled between the gate terminal 156 and the common return 110.
When transistor QHV 150 in the example of FIG. 1 is switched on, transistor QHV 150 conducts a current IB 172 through resistor 106 from constant voltage source 108, and the voltage VHV 154 between the drain terminal 152 and the source terminal 158 is much less than the voltage of constant voltage source VB 108. The voltage VHV 154 between the drain terminal 152 and the source terminal 158 is typically less than 5 volts when transistor QHV is switched on. When transistor QHV 150 is switched off, there is substantially zero current in resistor 106, and the voltage VHV 154 between the drain terminal 152 and the source terminal 158 is substantially the value of the constant voltage source VB 108. Therefore, in a typical application the voltage VHV 154 may switch between a few volts and several hundred volts.
Transistor QHV 150 in the example of FIG. 1 is known as a depletion mode device because it requires a non-zero voltage on the gate terminal 156 with respect to the source terminal 158 to stop conduction of drain current IB 172. The normal state of a transistor is generally considered to be a state with no signal between the gate and the source. That is, a depletion mode device is considered to be normally on because it may conduct current when it has zero volts between gate and source. Depletion mode HFET devices require a negative voltage that is more negative than a threshold value between gate and source to stop conduction of drain current IB 172. A positive voltage between gate and source as well as zero voltage between gate and source will allow a depletion mode device to conduct. In the example of FIG. 1, switching control voltage 162 switches to a negative value −VOFF that is more negative than the threshold voltage of transistor QHV 150 to switch transistor QHV 150 off. Conventional HFET devices are depletion mode devices.
When transistor QHV 150 is off and voltage VHV 154 is high, conductive components E3 116, E2 122, and E1 128 of the charge distribution structure 174 may accumulate a static charge from leakage currents in the electrically insulating material 104. The magnitude of the static charge that may accumulate without a static discharge system is influenced by the capacitance and the electrical resistance between the individual conductors, as well as by the average voltage that appears between the drain 152 and the gate 156 of transistor QHV 150. Electric fields that result from the accumulated static charge may reduce the breakdown voltage of transistor QHV 150. In the absence of a static discharge system, the accumulated charge may remain in the device for a time that may range from minutes to days. The static discharge system 102 shown in FIG. 1 may reduce the accumulated charge to negligible levels every time transistor QHV 150 turns on.
The circuit of the example static discharge system 102 in FIG. 1 includes transistors QD,3 178, Q3,2 134, Q2,1 138, and Q1,0 144. All transistors in FIG. 1 are depletion mode HFET devices. The transistors in the static discharge system 102 may have substantially lower breakdown voltages than high voltage transistor QHV 150 because resistors RD,3 114, R3,2 120, R2,1 126 and R1,0 132 form a voltage divider circuit that allows only a fraction of the high voltage VHV 154 to appear across any transistor in the static discharge system 102. As such, the transistors in the static discharge system 102 do not require charge distribution structures. Therefore, the low voltage transistors in the static discharge system 102 and the high voltage power transistor QHV 150 may be fabricated together on the same semiconductor wafer, with only transistor QHV 150 having a charge distribution structure.
FIG. 1 shows that the gate G0 148 of transistor Q1,0 144 in the static discharge system 102 is coupled directly to the gate G 156 of high voltage transistor QHV 150, and the source S0 164 of transistor Q1,0 144 is coupled to the source S 158 of high voltage transistor QHV 150 through the common return 110. Since transistors Q1,0 144 and QHV 150 are both depletion mode HFET devices with approximately the same threshold voltages, both transistors turn on and turn off at the same time.
When transistors QHV 150 and Q1,0 144 in the example of FIG. 1 turn on, all transistors turn on, and the drain-to-source voltages VHV 154, V1,0 146, V2,1 140, V3,2 136, and VD,3 176 will be typically less than five volts. Since transistor QHV 150 is substantially larger than the transistors in the static discharge structure 102, transistor QHV 150 conducts nearly all the current in resistor R 106. In other words, current IX 184 that goes to the charge distribution structure 174 and to the static discharge system 102 is negligible in comparison to the current IB 172 in transistor QHV 150.
The drain terminal D0 142 of transistor Q1,0 144 is coupled to conductive component E1 128 of the charge distribution structure 174. Since voltage VHV 154 will be typically less than five volts when transistor Q1,0 144 is on, V1,0 will be typically between zero and five volts when transistor Q1,0 144 is on. Therefore, when transistor Q1,0 144 turns on, static charge on conductive component E1 128 of the charge distribution structure 174 is discharged to the common return 110, and the voltage on the capacitance C1,0 130 is reduced to typically much less than five volts.
FIG. 1 shows transistor Q2,1 138 coupled between conductive components E2 122 and E1 128 of the charge distribution structure 174. The voltage between conductive components E2 122 and E1 128 is the drain-to-source voltage V2,1 140 of transistor Q2,1 138. The gate terminal of transistor Q2,1 138 is coupled to the common return 110. The source terminal of transistor Q2,1 138 is coupled to the drain terminal of transistor Q1,0 144, and to the gate terminal of transistor Q3,2 134, and to the conductive component E1 128 at a junction J1 166. As used in this disclosure, a junction is an electrical termination that is both a source terminal of a transistor and a drain terminal of a different transistor.
FIG. 1 also shows transistor Q3,2 134 coupled between conductive components E3 116 and E2 122 of charge distribution structure 174. The drain terminal of transistor Q3,2 134 is coupled to conductive component E3 116 of charge distribution structure 174 and to the source terminal of transistor QD,3 178 at a junction J3 180. The source terminal of transistor Q3,2 134 is coupled to the drain terminal of transistor Q2,1 138, and to the gate terminal of transistor QD,3 178, and to conductive component E2 122 of charge distribution structure 174 at a junction J2 168. The voltage between conductive components E3 116 and E2 122 of charge distribution structure 174 is the drain-to-source voltage V3,2 136 of transistor Q3,2 134. The gate terminal of transistor Q3,2 134 is coupled to the conductive component E1 128 of charge distribution structure 174, and to the source terminal of transistor Q2,1 138, and to the drain terminal of transistor Q1,0 144 at junction J1 166.
The example of FIG. 1 shows transistor QD,3 178 coupled between the drain terminal 152 of transistor QHV 150 and conductive component E3 116 of charge distribution structure 174. The voltage between the drain terminal 152 of transistor QHV 150 and conductive component E3 116 of charge distribution structure 174 is the drain-to-source voltage VD,3 176 of transistor QD,3 178. The drain terminal of transistor QD,3 178 is coupled to the drain terminal 152 transistor QHV 150. The source terminal of transistor QD,3 178 is coupled to the conductive component E3 116 of the charge distribution structure 174 and to the drain terminal of transistor Q3,2 134 at the junction J3 180. The gate terminal of transistor QD,3 178 is coupled to the conductive component E2 122 of charge distribution structure 174, and to the source terminal of transistor Q3,2 134, and to the drain terminal of transistor Q2,1 138 at the junction J2 168.
When transistor Q1,0 144 turns on to discharge capacitance C1,0 130 associated with conductive component E1 128, transistors Q2,1 138 and Q3,2 134 and QD,3 178 also turn on to discharge capacitances C2,1 124, C3,2 118, and CD,3 112 associated with the respective conductive components E2 122 and E3 116. Therefore, when high voltage transistor QHV 150 turns on, transistors of the static discharge system 102 set the voltages on the conductive components of the charge distribution structure 174 to approximately the potential of the common return 110, effectively removing accumulated static charge from the conductive components of the charge distribution structure 174.
When transistors QHV 150 and Q1,0 144 turn off and the voltage VHV 154 increases, the voltages on the conductive components E1 128, E2 122, and E3 116 of charge distribution structure 174 will increase to reach or exceed a magnitude that turns off transistors Q2,1 138, Q3,2 134, and QD,3 178. The voltage on conductive components E1 128, E2 122, and E3 116 of charge distribution structure 174 is limited to a fraction of the voltage VHV 154 as determined by the voltage divider circuit formed by the resistors RD,3 114, R3,2 120, R2,1 126, and R1,0 132 and/or capacitances C2,1 124, C3,2 118, and CD,3 112.
In a typical design, when transistor QHV 150 is off and VHV 154 is hundreds of volts, the voltage between any two adjacent conductive components of the charge distribution structure 174, will be between 10 and 200 volts. At the same time, the voltage between conductive component E1 128 of charge distribution structure 174 and the gate 156 of transistor QHV 150, will also be between 10 and 200 volts. In other words, the voltages VD,3 176, V3,2 136, V2,1 140, and V1,0 146 when transistor QHV 150 is off will be sufficient to turn off transistors QD,3 178 and Q3,2 134 and Q2,1 138. Therefore, when high voltage is present between the drain terminal 152 and the source terminal 158 of high voltage transistor QHV 150, all transistors in the static discharge system 102 will be off, providing minimum leakage current through the discharge system 102 and, therefore, minimum voltage drop across the load resistor R 106.
In other words, depletion mode devices in the static discharge system of the present invention are coupled to floating conductors of a charge distribution structure included in a semiconductor device. When the semiconductor device is on, the difference in voltage between the floating conductors is sufficiently low to turn on the depletion mode devices. When the semiconductor device is off, the difference in voltage between the floating conductors is sufficiently high to turn off the depletion mode devices.
FIG. 2 is an electrical schematic diagram 200 that shows how the example static discharge system of FIG. 1 may be expanded to accommodate a general charge distribution system that has N elements. In the example of FIG. 2, transistor QHV 250 is a high voltage HFET with a drain terminal 252, a source terminal 258, and a gate terminal 256. An internal charge distribution structure between the drain terminal 252 and the gate terminal 256 of transistor QHV 250 includes conductive components E1 228, E2 222, E3 216, E(N-1) 274, and EN 272 surrounded by an imperfect electrically insulating material 204.
In the general example of FIG. 2, the drain terminal D 252 of high voltage HFET QHV 250 is coupled to one end of a resistor R 206 and to the drain DN 288 of a transistor QD,N 278 in a static discharge system 202. The other end resistor R 206 is coupled to the positive terminal of a constant voltage source VB 208. The negative terminal of the constant voltage source VB 208 is coupled to a common return 210. The source terminal S 258 of high voltage HFET QHV 250 is also coupled to the common return 210. A variable voltage source 260 that produces a switching control voltage VGS 262 is coupled between the gate terminal G 256 of high voltage HFET QHV 250 and the common return 210. As in the example of FIG. 1, high voltage HFET QHV 250 operates as a switch that produces a switching voltage VHV 254 between the drain terminal 242 and the source terminal 258.
The general example of FIG. 2 also shows the static discharge system 202 that includes transistors Q1,0 244, Q2,1 238, Q3,2 234, Q4,3 282, QN,(N-1) 280, and QD,N 278. As in the example of FIG. 1, all transistors in the general example of FIG. 2 are depletion mode HFETs.
Transistor Q1,0 244 in the static discharge system of FIG. 2 has a gate terminal G 248 coupled to the gate terminal 256 of high voltage transistor QHV 250. Transistor Q1,0 has a source terminal S0 264 coupled to common return 210. Drain terminal D0 242 of transistor Q1,0 244 is coupled at a junction J1 266 to the source terminal of transistor Q2,1 238 and to the gate terminal of transistor Q3,2 234, and to the conductive component E1 228. The gate terminal of transistor Q2,1 238 is coupled to the common return 210. The drain terminal of transistor Q2,1 238 is coupled to the source terminal of transistor Q3,2 234, to the gate terminal of transistor Q4,3 282, and to the conductive component E2 222.
The voltage between conductive component E1 228 and the common return 210 is the voltage V1,0 246 between the drain terminal D0 244 and the source terminal S0 264 of transistor 244. The voltage between conductive components E2 222 and E1 228 is the voltage V2,1 240 between the drain terminal and the source terminal of transistor Q2,1 238. The voltage between conductive components E3 216 and E2 222 is the voltage V3,2 236 between the drain terminal and the source terminal of transistor Q3,2 234.
The general example of FIG. 2 shows the drain terminal DN 288 of transistor QD,N 278 coupled to the drain terminal D 252 of high voltage HFET QHV 250. The source terminal of transistor QD,N 278 in the general example of FIG. 2 is coupled to conductive component EN 272 and to the drain terminal of transistor QN,(N-1) 280 at a junction JN 276. The source terminal of transistor QN,(N-1) 280 is coupled to conductive component E(N-1) 274 and to the gate terminal of transistor QD,N 278 at a junction J(N-1) 268. The voltage between conductive components EN 272 and E(N-1) 274 is the voltage VN,(N-1) 286. The voltage between the drain D 252 of transistor QHV 250 and conductive component EN 272 is VD,N 284.
FIG. 2 shows that the static discharge system 202 may be applied to a charge distribution structure that is part of a high voltage HFET, the charge distribution structure having N conductive components designated as E1 through EN with conductive component E1 nearest the gate terminal of the HFET and conductive component EN nearest the drain terminal of the HFET. For non-trivial examples where N>1, the conductive components may be arranged so that for any two conductive components EK and E(K-1), the conductive component EK is nearer to the drain terminal of the high voltage HFET than the conductive component E(K-1), where K is an integer greater than one and less than or equal to N. Stated symbolically, in the preceding description K is an integer with 1<K≤N.
FIG. 2 also shows that low voltage HFETs of the static discharge system 202 may be coupled between the conductive components of the charge distribution structure to realize a circuit that has the drain terminal of a low voltage HFET QK,(K-1) coupled to conductive component EK, and the source terminal of the low voltage HFET QK,(K-1) coupled to conductive component E(K-1) at a junction J(K-1). The circuit further has the gate terminal of the low voltage HFET QK,(K-1) coupled to the junction J(K-2). For the special case of K=2, the circuit has the gate terminal of HFET QK,(K-1) coupled to the source terminal of the high voltage HFET. The general static discharge structure of FIG. 2 shows that a low voltage HFET QD,N may be coupled between the drain terminal of the high voltage HFET and the conductive component EN of the static discharge system.
FIG. 2 also shows that the general example circuit of a static discharge system 202 includes an additional low voltage HFET coupled with its drain terminal to conductive component E1, its source terminal coupled to the source terminal of the high voltage HFET, and its gate terminal coupled to the gate terminal of the high voltage HFET. Thus as illustrated by the general example of FIG. 2, a static discharge system for a high voltage HFET may include one low voltage HFET for every conductive component of a charge distribution structure that is part of the high voltage HFET. For the trivial case of only a single conductive component in the charge distribution structure (N=1 and K=0), the static discharge system reduces to the single transistor Q1,0 244.
Therefore, FIG. 2 shows an example of a semiconductor circuit comprising a three terminal semiconductor device 250, a surface charge distribution structure 204 having a plurality of conductors with a floating potential (228, 222, 216, 274, and 272), being capacitively coupled to a terminal of the semiconductor device, and a static discharge system 202 for removing charge that accumulates on the conductors when the semiconductor device is in a first (off) state while allowing charge to accumulate on the conductors when the semiconductor device is in a second (on) state.
It will be appreciated by those skilled in the art that the difference in voltage between the conductive components of the charge distribution structure 204 when the high voltage transistor QHV 250 is off depends on the number N of conductive components and the relative spacing between the conductive components. The voltage between adjacent conductive components may be reduced with larger N and reduced spacing. In examples where the voltage between adjacent conductive components of the charge distribution structure 204 is not large enough to turn off the transistors of the static discharge system 202, the gates of the low voltage HFETs may be coupled to conductive components that are farther away from the sources of their respective HFETs. In other words, the gate terminal of a low voltage HFET QK,(K-1) may be coupled to a junction J(K-2-M) where M is an integer greater than equal to one.
The transistors of the static discharge system may be fabricated using the same process as the semiconductor device that includes the charge distribution system. Moreover, the static discharge system may be fabricated on the same substrate as the semiconductor device that includes the charge distribution system and simultaneously with the semiconductor device that includes the charge distribution system.
FIG. 3 shows the salient features of a general example of a static discharge system in accordance with the teachings of the present invention. In particular, FIG. 3, which is not drawn to scale, is a cross-sectional view 300 through one part of a static discharge system that includes several heterostructure field effect transistors (HFETs). The schematic diagram of the static discharge system 202 of FIG. 2 is representative of the cross section of the semiconductor construction illustrated in FIG. 3.
The example static discharge system illustrated in FIG. 3 comprises a first active layer 356 and a second active layer 352. The first active layer 356 is typically disposed over a substrate (not shown in FIG. 3) that may be formed from various materials such as sapphire (Al2O3), silicon (Si), or silicon carbide (SiC) as is known in the art. Various techniques of fabrication may call for layers of other materials to be disposed between the substrate and the first active layer 356 to facilitate the construction of the device.
The first active layer 356 in the example of FIG. 1 is comprised of gallium nitride (GaN). In other examples, different semiconductor materials containing nitride compounds of other elements from Group III of the Periodic Table of the Elements may comprise the first active layer 356.
The second active layer 352 in the example of FIG. 3 is comprised of aluminum gallium nitride (AlGaN). In other examples, different Group III nitride semiconductor materials such aluminum indium nitride (AlInN) and aluminum indium gallium nitride (AlInGaN) may comprise the second active layer 352. The material of the second active layer 352 may be a non-stoichiometric compound. In such materials, the ratios of the elements are not easily represented by ordinary whole numbers. For example, the second active layer 352 may be a non-stoichiometric compound of a Group III nitride semiconductor material such as AlXGa1-XN where 0<X<1.
In the example of FIG. 3, a conductive source contact 326, a conductive drain contact 348, and conductive gate contacts 328, 332, 336, 340, and 346 are disposed over the second active layer 352. Also disposed over the second active layer 352 are conductive junction contacts 330, 334, 338, 342, and 344. As used in this disclosure, a junction contact joins the source contact of a transistor to the drain contract of a neighboring transistor.
Also in the example of FIG. 3, a gate dielectric layer 350 separates the gate contacts 328, 332, 336, 340, and 346 from the second active layer 352. A dielectric is an electrically insulating material that can be polarized by an external electric field. Polarization orients bound electric charge in the material to make the magnitude of the electric field inside the material less than the magnitude of the external electric field.
The source, gate, junction, and drain contacts shown in the example of FIG. 3 are parts of several individual transistors of a static discharge structure 202 as described in the electrical schematic of the general example of FIG. 2. The source S0 326, the gate G0 328, and junction J1 330 are respectively source, gate, and drain terminals of HFET Q1,0 302. Similarly, junction contact J1 330, gate contact G1 332, and junction contact J2 334 are respectively the source, gate and drain terminals of HFET Q2,1 304. Junction contacts J2 324, J3 338, J4 342, and JN 344 with gate contacts G2 336, G3 340, GN 346 plus drain contact DN 348 form HFETs Q3,2 306, Q4,3 308, and QD,N 310.
The example HFETs depicted in FIG. 3 are also known as high electron mobility transistors (HEMTs). In the HEMTs of FIG. 3, the second active layer 352, which generally has a higher bandgap than the first active layer 356, gives rise to a layer of electric charge 354 in the first active layer 356 near the second active layer 352. The layer of electric charge 354 defines a lateral conductive channel. The first active layer 356 is sometimes called the channel layer. The second active layer 352 is sometimes called the barrier layer or the donor layer. The layer of electric charge 354 which defines the lateral conductive channel is sometimes called a two-dimensional electron gas (2DEG) because electrons, trapped in the quantum well that results from the difference in the bandgaps, are free to move in two dimensions but are tightly confined in the third dimension.
In the operation of an HFET, for example Q1,0, electric charge in the two-dimensional electron gas 354 moves in a channel between the source contact S0 326 and the drain contact J1 330 to become a current in an external circuit. The electric charge, and hence the current, is controlled by a voltage from an external circuit that is electrically connected between the gate contact G0 328 and the source contact S0 326.
The example static discharge structure of FIG. 3 shows electrical connections between source, gate, and junction contacts. As used in this disclosure, an electrical connection is an ohmic connection. An ohmic connection is one in which the relationship between the voltage and the current is linear and symmetric for both directions of the current. For example, two metal patterns that contact each through only metal are electrically connected. In contrast, the junction contact J1 330 and the source contact S0 330 are not electrically connected in the example of FIG. 3 because any connection between these contacts is through a channel in the semiconductor and controlled by the gate contact G0 328. Similarly, the gate contact G0 328 is not electrically connected to the semiconductor 352 under the gate contact G0 328, and the gate contact G0 328 is not electrically connected to the 2DEG 354 when a gate dielectric 350 is used to insulate the gate contact G0 328 from the semiconductor 352 beneath.
A passivation layer 312 of a dielectric material is disposed around the contacts and above the gate dielectric layer 312 in the example of FIG. 3. The passivation layer 312 provides stability of the electrical characteristics of the device by isolating the surface of the device from electrical and chemical contaminants in the environment. The passivation layer 312 may be deposited in more than one step to allow for metal to be embedded within the passivation material.
In other examples that use different arrangements of passivation layers, the gate dielectric layer 350 is optional. In examples without the gate dielectric layer 350, the gate contacts G0 328, G1 332, G2 336, G3 340, and GN 346 each form a Schottky contact to the second active layer 352. A Schottky contact is a junction between a metal and a semiconductor that conducts current between the metal and the semiconductor in only one direction. Transistors without the gate dielectric layer are sometimes called Schottky gate transistors. An example of a construction that uses an optional gate dielectric layer is presented later in this disclosure.
The passivation 312 in the example of FIG. 3 is transparent, giving visibility to objects that lie behind the plane of the cross-section. For example, the metal 322 that makes an electrical connection between junction contact J1 330 and gate contact G2 336 is in a plane behind metal 314 and metal 316. Metal 324 is behind metal 316 and metal 318. The metal 318 that makes an electrical connection to junction contact J4 342 would make an electrical connection with a gate G5 not shown in the example of FIG. 3. The metal 320 that makes an electrical connection to the gate contact GN 346 would make an electrical connection with a junction contact J(N-1) not shown in the example of FIG. 3.
FIG. 4 is a top view 400 that shows the example static discharge system of FIG. 3 with a high voltage power HFET 480. The static discharge system 402 and the high voltage power HFET 480 in the example of FIG. 4 may be fabricated as separate devices on the same semiconductor substrate as will be described in detail later in this disclosure. The passivation material in the example static discharge system of FIG. 3 is transparent in the view of FIG. 4, providing visibility to the gate dielectric 482 and to various conductive components. The elements of FIG. 4 are not shown to scale.
The high voltage power HFET 480 in the example of FIG. 4 has a significant section of its interior removed to emphasize that the high voltage power HFET 480 is typically substantially larger than the static discharge system 402. In practice, the width of the conducting channel of each transistor in the static discharge system 402 may be less than 2% of the width of conducting channel in the high voltage power HFET 480. The transistors in the static discharge system 402 must be only large enough to conduct the accumulated charge from the components of the charge distribution structure during the transitions in voltage on the high voltage power HFET 480. The high voltage power HFET 480 includes a source contact 462, a gate contact 464, a drain contact 478, and a charge distribution structure with conductive components E1 468, E2 470, E3 472, E4 474, and EN 476.
The top view of the example static discharge system 402 shown in FIG. 4 reveals the layout of the metal that makes electrical connections between contacts of the low voltage HFETs. The example of FIG. 4 shows metal 438 connecting source contact S0 404 to gate contact G1 414, metal 434 connecting junction contact J1 412 to gate contact G2 418, and metal 440 connecting junction contact J2 416 to gate contact G3 424. Metal 436 would connect junction contact J3 422 to gate contact G4 not shown in FIG. 4. Metal 442 would connect junction contact J4 426 to gate contact G5 not shown in FIG. 4. Metal 444 would connect gate contact GN 430 to junction contact J(N-1) not shown in FIG. 4.
The horizontal sections of the metal 434, 436, 438, 440, and 444 of the static discharge structure shown in FIG. 4 connect at their ends to vertical sections of metal not visible in FIG. 4. The vertical sections of metal connect to the contacts directly below the ends of the horizontal sections of metal. For example, horizontal metal section 434 connects to a vertical metal section 410 at one end that in turn connects to junction contact J1 412, and horizontal metal section 434 connects to a vertical metal section 418 at its other end that in turn connects to gate contact G2 420.
The example of FIG. 4 also shows connections between the static discharge system 402 and the high voltage HFET 480. Metal 446 connects the source contact S0 404 of the static discharge system 402 to the source contact 462 of the high voltage HFET 480. Metal 448 connects the gate contact G0 408 of the static discharge system 402 to gate contact 464 of the high voltage HFET 480. Metal 450 connects the junction contact J1 412 of the static discharge system 402 to the conductive component E1 468 of the charge distribution structure of the high voltage HFET 480. Metal 452 connects the junction contact J2 416 and to conductive component E2 470 of the charge distribution structure of the high voltage HFET 480. Metal 454 connects the junction contact J3 422 to the conductive component E3 472 of the charge distribution structure of the high voltage HFET 480. Metal 456 connects the junction contact J4 426 to the conductive component E4 474 of the charge distribution structure of the high voltage HFET 480. Metal 458 connects the junction contact JN 428 to the conductive component EN of the charge distribution structure of the high voltage HFET 480. Metal 460 connects the drain contact DN 432 to the drain contact 478 of the high voltage HFET 480.
FIG. 5A is a perspective view 500 showing the relative positions of various parts and features of an example static discharge system including a high voltage power HFET that has a charge distribution structure in accordance with the teachings of the present invention. FIG. 5B is a different perspective view 560 of the static discharge system shown in FIG. 5A. The example semiconductor device shown in FIG. 5A and FIG. 5B is similar to the example device described the in the cross-section view of FIG. 3 and in the top view of FIG. 4, and is represented in the electrical schematic diagram of FIG. 1
The perspective views of FIG. 5A and FIG. 5B, and all other perspective views presented in this disclosure may not be drawn to scale. Some dimensions may have been exaggerated to show details that would not be visible in a scaled drawing.
FIG. 5A and FIG. 5B illustrate how a static discharge system may be assembled with a high voltage HFET on the same substrate. Low voltage HFETs for the static discharge system may be fabricated with the same process and at the same time as the high voltage HFET. The static discharge system and the high voltage HFET may be fabricated as independent devices that are electrically coupled only through conductors that span the distance between the devices on the substrate.
Metal that connects contacts of the low voltage transistors and that connects the low voltage transistors to the charge distribution structure of the high voltage HFET may deposited at the same time as the metal that forms the charge distribution structure of the high voltage HFET. Portions of some layers of the example device have been removed in FIG. 5A and FIG. 5B to expose underlying features in greater detail. All layers would normally cover the entire device, ending at the same vertical plane on each side of the device.
FIG. 5A and FIG. 5B show the structures of an example static discharge system 552 and a portion 554 of a high voltage HFET. The example static discharge system 552 and the portion of the high voltage HFET 554 are on the same substrate 536, but they have independent active areas. The active areas are the regions of the active layers under the contacts that take part in the operation of the devices. The portion of the HFET 554 visible in the perspective drawings of FIG. 5A and FIG. 5B may represent typically less than 2% of the high voltage transistor.
In the example illustrated in FIG. 5A and FIG. 5B, the first active layer 534 of the static discharge system 552 and the first active layer 535 of the high voltage HFET 554 are of the same material composition, but the two first active layers are spatially separated from each other by a region where there is no first active layer. Also in the example illustrated in FIG. 5A and FIG. 5B, the second active layer 532 of the static discharge system 552 and the second active layer 540 of the high voltage HFET 554 are of the same material composition, but the two second active layers are spatially separated from each other by a region where there is no second active layer. Therefore, the transistors of the static discharge system 552 may operate independently from the high voltage HFET 554 even though all the devices are fabricated on the same substrate 536.
Consistent with the cross-section representation of FIG. 3 and the top view of FIG. 4, the perspective views of FIG. 5A and FIG. 5B show for the static discharge system a first active layer 534, a second active layer 532, and a dielectric layer 516 above a substrate 536. FIG. 5A and FIG. 5B also show for the high voltage HFET 554 a first active layer 535, a second active layer 540, and the dielectric layer 516 above the substrate 536. In the example of FIG. 5A and FIG. 5B, the dielectric layer 516 is continuous over the area of the static discharge system 552, the high voltage HFET 554, and the region where there are no active layers. FIG. 5A and FIG. 5B show conductive contacts for sources, gates, and drains of four low voltage HFETs and one high voltage HFET.
The example of FIG. 5A and FIG. 5B shows the gate dielectric 516 deposited over the second active layer 532 in the region of the static discharge system 552 and over the second active layer 540 in the region of the high voltage HFET 554. The gate dielectric layer 516 visible in FIG. 5A in the region of the static discharge system 552 corresponds to the region 350 in the cross-sectional view of the example of FIG. 3. The passivation 312 shown in the example cross-sectional view of the example of FIG. 3 corresponds to a first passivation layer 510 and a second passivation layer 502 in the example of FIG. 5A and FIG. 5B to indicate that the example the passivation is deposited in two steps of the fabrication process.
Contacts for terminals of the high voltage HFET 554 are visible in the perspective view of FIG. 5B as the source 550, the gate 562, and the drain 564. The perspective views of FIG. 5A and FIG. 5B also show conductive components E1 548, E2 546 and E3 542 of a charge distribution structure within the region of high voltage HFET 554. The conductive components E1 548, E2 456, and EN 542 may be formed from an electrically conducting material or materials such as one or more metals, heavily doped semiconductors, and the like.
Horizontal components of the charge distribution structure are fabricated from conductive material of appropriate thickness, appearing as stripes in a conventional two-dimensional view from the top. In one example, the thickness of the stripes may be approximately the same dimension as the width of the stripes. While the examples discussed herein refer to the horizontal components as stripes, more generally the horizontal components may be elongate members having a wide variety of different cross-sectional shapes.
FIG. 5A and FIG. 5B reveal that the conductive components E1 548, E2 546 and E3 542 of the charge distribution structure are formed from horizontal top stripes and horizontal bottom stripes that are connected with vertical conducting pillars through the first passivation layer 510. Contacts for terminals of low voltage HFETs in the region of the static discharge structure 552 are visible in the perspective view of FIG. 5A as source contact S0 530, drain contact D3 514, junction contacts J1 526, J2 522, J3 518, and gate contacts G0 528, G1 524, G2 520, and G3 512.
The contacts of the transistors in the example illustrated in the perspective views of FIG. 5A and FIG. 5B are connected with deposits of metal. For example, the metal 504 connects junction contact J1 526 to gate contact G2 520. The metal 506 connects junction contact J2 522 to gate contact G3 512. The metal 508 connects gate contact G1 524 to source contact S0 530. The metal 538 connects source contact S0 530 of the static discharge system 552 to source contact 550 of the high voltage HFET 554. The first passivation layer 510 and the second passivation layer 502 are removed in the perspective views of FIG. 6A and FIG. 6B to reveal more of the metal in the examples of FIG. 5A and FIG. 5B.
FIG. 6A is a perspective view 600 that shows the example static discharge system of FIG. 5A with passivation layers 510 and 502 removed to show additional detail. FIG. 6B is a different perspective view 660 of the example static discharge system shown in FIG. 6A. FIG. 6A and FIG. 6B reveal the conductors under the passivation and above the gate dielectric 516 that occupies the space between the region of the static discharge system 552 and the high voltage HFET 554 on the substrate 536.
FIG. 7 is an example flow diagram illustrating generally a sample flow of operations in an example process to construct the example semiconductor device illustrated in FIG. 5A and FIG. 5B in accordance with the teachings of the present invention. After starting in block 705, structures for high electron mobility transistors (HEMTs) are formed in block 710 with active areas and ohmic contacts for the drains and the sources as is known in the art. A gate dielectric layer is then formed over the active areas in step 715.
After the gate dielectric layer is formed in block 715, metallization for the gates of the HEMTs, metallization for the bottom stripes of the charge distribution structure, and metallization for the connections between the high voltage HFET and the transistors of the static discharge system are formed over the gate dielectric layer in block 720. Next, in block 725, a first passivation layer is formed over the metal previously deposited.
Vias are then formed through the first passivation layer in block 730 where it is desired to join the bottom stripes with the top stripes to form components of the charge distribution structure. Vias are also formed over appropriate portions of the transistors in the static discharge system to provide connections between them as described above. Metal is then deposited in block 735 on the surface of the first passivation layer and through the vias in the first passivation layer to form top stripes of the charge distribution structure that are joined to bottom stripes of the charge distribution structure. Metal is also deposited on the surface of the first passivation layer and through the vias in the first passivation layer to form connections between the static discharge transistors. The process ends with the formation of a second passivation layer in block 740.
FIG. 8A is a perspective view 800 showing the relative positions of various parts and features of another example static discharge system 852 sharing a substrate 836 with a high voltage HFET 854 (see FIG. 8B) that includes a charge distribution structure in accordance with the teachings of the present invention. FIG. 8B is a different perspective view 880 of the device shown in FIG. 8A. The example static discharge system 852 shown in FIG. 8A and FIG. 8B has an additional layer of passivation 810 that allows an optional gate dielectric layer 816, whereas the gate dielectric layer 516 is mandatory in the structure illustrated in FIG. 5A, FIG. 5B, FIG. 6A, and FIG. 6B. In examples without the optional gate dielectric layer 816, the gate contact 862 forms a Schottky contact to the second active layer 840, and the passivation layer 810 insulates the components E1 868, E2 870, and E3 872 of the charge distribution structure from the second active layer 840.
FIG. 8A and FIG. 8B illustrate how a static discharge system 852 may be assembled on the same substrate as a high voltage HFET 854 during the fabrication of the HFET 854 in accordance with the teachings of the present invention. As in FIG. 5A and FIG. 5B, portions of some layers of the example device have been removed in FIG. 8A and FIG. 8B to expose underlying features in greater detail.
Similar to the views of FIG. 5A and FIG. 5B, the perspective views of FIG. 8A and FIG. 8B show a static discharge system 852 and a portion of a high voltage HFET 854. The perspective views of FIG. 8A and FIG. 8B show for the static discharge system 852 a first active layer 834, a second active layer 832, and an optional dielectric layer 816 above a substrate 836. FIG. 8A and FIG. 8B also show for the high voltage HFET 854 a first active layer 835, a second active layer 840, and the optional dielectric layer 816 above the substrate 836. FIG. 8A and FIG. 8B show that the two active areas of the static discharge system 852 are spatially separated from the two active areas of the high voltage HFET 854 by a region where there are no active layers.
In the example of FIG. 8A and FIG. 8B, the optional dielectric layer 816 is continuous over the area of the static discharge system 852, the high voltage HFET 854, and the region where there are no active layers. FIG. 8A and FIG. 8B show conductive contacts for sources, gates, and drains of four low voltage HFETs and one high voltage HFET as illustrated in the schematic diagram of FIG. 1.
The example of FIG. 8A shows the optional gate dielectric 816 deposited over the second active layer 832 in the region of the static discharge system 852, and FIG. 8B shows the optional gate dielectric 816 deposited over the second active layer 840 in the region of the high voltage HFET 854. The example of FIG. 8A and FIG. 8B also shows a first passivation layer 810, a second passivation layer 802, and a third passivation layer 876 deposited over the optional gate dielectric 816. In examples that do not use the gate dielectric 816, gate contact 862 is deposited directly on the second active layer 840 to form a Schottky contact.
Contacts for terminals of the high voltage HFET 854 are visible in the perspective view of FIG. 8B as the source 850, the gate 862, and the drain 864. The perspective views of FIG. 8A and FIG. 8B also show conductive components E1 848, E2 846 and E3 842 of a charge distribution structure within the region of high voltage HFET 854. The conductive components E1 848, E2 846, and EN 842 may be formed from an electrically conducting material or materials such as one or more metals, heavily doped semiconductors, and the like.
FIG. 8A and FIG. 8B reveal that the conductive components E1 848, E2 846 and E3 842 of the charge distribution structure are formed from horizontal top stripes and horizontal bottom stripes above and below passivation layer 802, respectively. Contacts for terminals of low voltage HFETs in the region of the static discharge structure 852 are visible in the perspective view of FIG. 8A as source contact S0 830, drain contact D3 814, junction contacts J1 826, J2 822, J3 818, and gate contacts G0 828, G1 824, G2 820, and G3 812.
The contacts of the transistors in the example illustrated in the perspective views of FIG. 8A and FIG. 8B are connected with deposits of metal. For example, the metal 804 connects junction contact J1 826 to gate contact G2 820. The metal 806 connects junction contact J2 822 to gate contact G3 812. The metal 808 connects gate contact G1 824 to source contact S0 830. The metal 808 also connects source contact S0 830 of the static discharge system 852 to source contact 850 of the high voltage HFET 854.
The metal 874 connects drain contact D3 814 of the static discharge system 852 to drain contact 864 of the high voltage HFET 854. The metal 872 connects junction contact J3 818 of the static discharge system 852 to conductive component E3 842 of the high voltage HFET 854. The metal 870 connects junction contact J2 822 of the static discharge system 852 to conductive component E2 846 of the high voltage HFET 854. The metal 868 connects junction contact J1 826 of the static discharge system 852 to conductive component E1 848 of the high voltage HFET 854. The metal 866 connects gate contact G0 828 of the static discharge system 852 to the gate 862 of the high voltage HFET 854.
FIG. 9A is a perspective view 900 that shows the example static discharge system of FIG. 8A with passivation layers 802 and 876 removed to show additional detail. FIG. 9B is a different perspective view 980 of the example static discharge system shown in FIG. 9A. FIG. 9A and FIG. 9B reveal the metal above the first passivation layer 810.
FIG. 10 is an example flow diagram illustrating generally a sample flow of operations in an example process to construct the example semiconductor device illustrated in FIG. 8A, FIG. 8B, FIG. 9A, and FIG. 9B in accordance with the teachings of the present invention. After starting in block 1005, structures for high electron mobility transistors (HEMTs) are formed in block 1010 with active areas and ohmic contacts for the drain and the source as is known in the art. An optional gate dielectric layer may then be formed over the active area in block 1015. Block 1015 is skipped in examples that do not use the optional gate dielectric layer.
After the completion of the operation in block 1015 in examples that use the optional gate dielectric layer, metallization for the gates is then formed in block 1020. Next, a first passivation layer is formed over the metal for the gates and the contacts for the drains and sources in block 1025. The formation of the first passivation layer may include removal of passivation material to form channels for the bottom stripes of the charge distribution structure. Then in block 1030 metal is deposited for the bottom stripes of the charge distribution structure, followed by a second passivation layer in block 1035.
Vias are then formed through the second passivation layer in block 1040 where it is desired to join the bottom stripes with top stripes to form components of the charge distribution structure and to make connections to the contacts of the static discharge structure. Metal is then deposited in block 1055 on the surface of the second passivation layer and through the vias in the second passivation layer to form top stripes of the charge distribution structure that are joined to bottom stripes of the charge distribution structure, and to form conductors that connect contacts of the static discharge system to contacts of the high voltage HFET. The process ends with the formation of a third passivation layer in block 1050.
While the teachings of the present invention have been described above in the context of an HFET, these teachings may be used with other semiconductor devices as well. For example, the surface charge distribution structure of the present invention may be used with an inverted HFET, which is similar to an HFET except that the donor layer is formed below the channel layer instead of above the channel layer as in an HFET.
The above description of illustrated examples of the present invention, including what is described in the Abstract, are not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible without departing from the broader spirit and scope of the present invention. Indeed, it is appreciated that the specific example voltages, currents, frequencies, power range values, times, etc., are provided for explanation purposes and that other values may also be employed in other embodiments and examples in accordance with the teachings of the present invention.
These modifications can be made to examples of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
Ramdani, Jamal, Kudymov, Alexey
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